1. Field of the Invention
The present invention relates to a low-melting point glass sealed semiconductor device containing a semiconductor chip which is die-bonded to a ceramic substrate by use of a thermosetting mounting agent.
2. Description of the Related Art
As is shown in FIG. 1, a conventional low-melting point glass sealed semiconductor device comprises: a ceramic substrate 1 coated with low-melting point glass 3 and serving as a cap; and a ceramic substrate 2 coated with low-melting point glass 4 and having a chip 5 thereon. The substrates 1 and 2 are overlaid with each other, with their glass-coated sides overlapped, and are then subjected to heat treatment. After melted by this heat treatment, the glass coated on the substrates 1 and 2 sets or hardens, thereby sealing the chip 5 on the ceramic substrate 2.
The conventional low-melting point glass sealed semiconductor device shown in FIG. 1 will be described in more detail. The semiconductor chip 5 is located on the predetermined position of the ceramic substrate 2. The semiconductor chip 5 is adhered to the ceramic substrate 2 by use of a thermosetting mounting agent (not shown). Normally, a polyimide-based agent is used as this thermosetting mounting agent, since it enables efficient work and provides reliable adhesion and since it is lower in price than gold paste. A lead frame 6 is located on low-melting point glass 4. It is electrically connected to the semiconductor chip 5 by wires 7. The ceramic substrate 1 serving as a cap is adhered to the other ceramic substrate 2. Between these ceramic substrates 1 and 2, the lead frame 6 is sandwiched and hermetically sealed. It should be noted that the thermosetting mounting agent used in the semiconductor device has a characteristic of decomposing into a gas at a high temperature.
The low-melting point glass sealed semiconductor device mentioned above is manufactured as follows. First, the ceramic substrate 2 is coated with low-melting point glass 4 in a similar manner to that in a printing process, except for the portion on which the semiconductor chip 5 is to be mounted. Next, the lead frame 6 is mounted on the low-melting point glass 4, and the chip 5 is mounted on the portion which is not coated with the glass, by use of the thermosetting mounting agent. Thereafter, the lead frame 6 and the semiconductor chip 5 are electrically connected together by wires 7. Next, the low-melting point glass 3 on ceramic substrate 2 and the low-melting point glass 4 on ceramic substrate 3 are heated at a temperature higher than their melting points. In the state where the glass is melted, the ceramic substrates 1 and 2 are overlaid with each other and hermetically sealed.
However, the conventional semiconductor device mentioned above cannot be fabricated at high yield and is not very reliable with respect to the sealed characteristic. This is because the thermosetting mounting agent it uses is likely to decompose into a gas at a high temperature. If the thermosetting mounting agent hardens, gas voids are generated in the low-melting point glass 3, 4. In addition, the gas spreads along the surface of the lead frame 6, so that a tunnel-like void extending from the interior of the glass to the outside of the semiconductor device is formed, resulting in defective adhesion.